Twenty children with different malignancies were vaccinated using an Oka-strain varicella vaccine (Varilrix) obtained from Smith Kline-RIT. Until now, no serious side effects to the vaccine have been observed and none of the vaccinees have had a varicella infection during this period. Nine children have been tested for cell-mediated immune responses against varicella antigen. Antibodies against varicella have been measured using an enzyme-linked immunosorbent assay technique. Tests were performed 4 weeks, 6 and 12 months after vaccination; at 4 weeks, 85% of the children had seroconverted, but by 12 months, those patients who were negative before vaccination had no detectable antibodies. While the blastogenic lymphocyte response to varicella antigen gradually increased during the first year after vaccination, it was much lower than after natural infection. The results indicate the need for revaccination, probably within 6 months after the first vaccination.